Xerographic development

ABSTRACT

Apparatus is herein disclosed for developing a latent electrostatic image on the surface of a moving image retaining member. An extended electrode is positioned adjacent the moving surface to form a development zone therebetween. Means are provided to move a closely packed flow of two component developer material moving through the development zone in contact with the photoreceptor wherein toner particles are electrostatically exchanged between the developer material and the imaged areas on the photoreceptor. Biasing means are provided to place the backing electrode at a potential to enhance the developability of the system.

United States Patent 1 Aug. 22, 1972 Stover [541 XEROGRAPHIC DEVELOPMENT [72] Inventor: Raymond W. Stover, Webster, NY.

[73] Assignee: Xerox Corporation, Stamford,

Conn.

[22] Filed: July 10, 1970 [21] Appl. No.: 53,723

[52] US. Cl ..118/637, 117/175, 1l8/DlG. 24 [51] Int. Cl. ..G03g 13/00 [58] Field of Search ..118/637; 117/175 [56] References Cited UNITED STATES PATENTS 2,952,241 9/1960 Clark et al ..1 18/637 3,448,724 6/1969 Chawda et a1 ..1 18/637 3,545,968 12/1970 Masamichi Sato ..96/l 3,542,579 11/1970 Gundlach ..1 17/175 3,216,844 11/1965 King ..ll7/17.5 3,503,776 3/1970 Gundlach ..1 18/637 Primary Examiner-Mervin Stein Assistant Examiner-Leo Millstein Attorney-Paul M. Enlow, James J. Ralabate, Donald F. Daley and Thomas J. Wall [57] ABSTRACT 13 Claims, 3 Drawing Figures PATENTED M1822 I972 FIG. 3

PRIOR ART FIG. 2

INVENTOR. RAYMOND W. STOVE-IR A TTORNEV XEROGRAPHIC DEVELOPMENT BACKGROUND OF THE INVENTION This invention relates generally to xerographic development and, in particular, to a two component development system.

More specifically this invention relates to a two component development system suitable for use in an automatic xerographic reproducing machine. In the art 'of xerography, as originally disclosed by Carlson in US. Pat. No. 2,297,691, a photosensitive plate, which consists of a photoconductive coating placed upon a conductive backing, is uniformly charged and the plate surface then exposed to a light image of the original subject matter to be reproduced. Under the influence of the light image, the photoconductive surface is caused to become conductive in the light struck or non-imaged areas to selectively dissipate the charge thereon to record a latent electrostatic image of the original subject matter. The latent image is generally made visible by contacting the unexposed or charged areas with an oppositely charged, finely divided, electroscopic marking powder. Areas of high charge concentration on the plate surface are recorded as images of high toner density while the areas of weaker charge concentration are recorded as proportionately less dense images. After development, the visible powder image may be transferred to a final support sheet, such as paper or the like, and the image affixed thereto to form a permanent record of the original.

The classical two component development technique has found wide acceptance in automatic xerography primarily because of the simplicity of operation and the ease of material handling afforded. Most two component development systems can be characterized by the nature of the flow mechanism involved. For example, cascade development, as originally disclosed by Walkup in US. Pat. No. 2,573,881, involves a relatively turbulent flow of development material through the development zone while the flow associated with C- shell development, as taught by Gundlach in US. Pat. No. 3,503,776 is extremely gentle. Where material life is a consideration, the more gentle type system is preferred. However, because quiescence was achieved primarily by limiting the developer flow rate, this type of system invariably produced poor development of low density image and was extremely limited as to speed at which development could be accomplished.

SUMMARY OF THE INVENTION It is therefore an object of this invention to improve xerographic development.

A further object of this invention is to provide a relatively gentle development system capable of developing images over a wide range of input speeds.

A still further object of this invention is to improve the low density developability of a two component development system utilizing a gentle developer flow mechanism.

These and other objects of the present invention are attained by positioning an extended electrode adjacent to a moving photoconductive surface, packing the zone between the electrode and the moving photoconductor with a moving bed of two component developer material whereby latent electrostatic images on the photoreceptor move in contact with the two component material and are developed.

For a better understanding of this invention as well as other objects and further features thereof, reference is had to the following, detailed description of the invention to be read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS DESCRIPTION OF A PREFERRED EMBODIMENT It is believed that a brief discussion of the flow mechanism relating to the heretofore mentioned Gundlach C-shell development system is warranted at this time in order to more fully appreciate the teachings of the present invention. The basic C-shell environment is illustrated in FIG. 3. A drum type photoreceptor 10 is arranged to move upwardly through a clam shell type developer housing 19 as the drum is rotated in the direction indicated about shaft 12. A bath of closely packed two component developer material 21 is supported within the developer housing in contact with the photoreceptor surface 11. Due to the frictional forces involved, a relatively thin layer of the developer material at the drum developer interface is moved in an upward direction at approximately drum speed. This induced upward flow of developer material creates a void at the start of the contact zone that is quickly filled by developer material moving down the backside of the system more or less as a unit. A circulating flow of developer material is thus established within the developer housing. This flow is illustrated by the broken line 40 shown in FIG. 3.

As can be seen, C-shell development involves a continuous two component flow process. Theoretically, properly toned developer beads are delivered to the start of the contact or development zone (S) and the material carried upwardly in contact with the image bearing photoreceptor surface. Although the exact development mechanism associated with this process is not fully understood at this time, it is believed that development is accomplished as the toner particles are electrostatically stripped from the carrier beads as they come under the influence of the force fields associated with the more highly charged image areas on the plate surface. As the photoreceptor is drawn from the developer bath, the developer material in contact therewith is returned to the backside of the system where the material is replenished with toner prior to being once again delivered to the start of the active development zone.

C-shell development has certain discernable advantages over many of the more conventional two component development systems. Because of the quiescent flow mechanism involved, both developer life and photoreceptor life is considerably extended. Deposition of randomly dispersed unwanted toner particles in nonimaged or background areas is also minimized because clouds of free toner particles are not generated during development. On the other hand, it has been found that the movement of developer material through the C- shell system is restricted primarily due to relatively slow movement of material as it moves down the backside of the system. A pronounced reduction in the systems developability is noted when high speed copying or extended development runs are accomplished. Another disadvantage which may be attributed to the systems flow characteristics is its inability to completely develop low density images.

The method and apparatus of the instant invention preserves all the advantages of the Gundlach C-shell system while at the same time overcoming many of the disadvantages associated therewith.

Although the apparatus of the present invention is well suited for use in a wide variety of automatic devices, it is herein shown embodied in a drum type automatic xerographic reproducing machine. It should be clear, however, to one skilled in the art that the developing method and apparatus of the present invention are not limited to this particular embodiment and the embodiment disclosed is for purposes of illustration only. Referring now specifically to FIG. l, a drum having a photoconductive surface ill thereon is mounted on shaft 112 and the shaft rotatably supported in the side frames (not shown) of the automatic reproducing machine. The major xerographic processing components are conveniently positioned about the drum periphery and serve to act on the photoconductive surface as the drum is rotated in the direction indicated.

In general, the several xerographic processing stations may be described functionally as follows: a charging station A, wherein a uniform electrostatic charge is deposited on the photoconductive surface by a corona discharge device 13 extending transversely across the photoconductive surface of the drum; an exposure station B, wherein a light or radiation pattern of the original subject matter to be reproduced is projected onto the photoconductor to dissipate the charge thereon thus forming a latent electrostatic image of the original on the drum surface; a development station C, wherein the latent electrostatic image is developed by means of a two component electroscopic developer material including carrier beads and relatively smaller dry powder particles being capable of making the electrostatic image visible when brought into contact therewith; an image transfer station D, wherein the developed powder image is transferred from the photoconductive surface to a moving web 14 of final support material advanced at drum speed between supply spool 15 and take up spool 16; and a drum cleaning station E, wherein residual powder developer material remaining on the photoconductive surface after the transfer operation is removed therefrom by means of a fiberous cleaning member 17.

A heat fusing device 18 is positioned in fixing station F. The heater is positioned down stream from the image transfer station and is arranged to act upon the web to fix images thereon prior to storing the web on spool 16. Although a heat fusing device is shown in this preferred embodiment, any suitable fixing means may be used without departing from the teachings of the present invention.

A developer housing 20 is shown positioned adjacent to drum 10 in processing station C. As illustrated, the photoconductive surface on the drum is arranged to continually move through an opening provided in one of the sidewalls of the housing as the drum is rotated in the direction indicated. Developer seals (not shown) are provided to prevent developer material from escaping from the housing. An extended electrode 25 is mounted in the housing and is electrically isolated from the other machine component by known means. The upper portion of the electrode cooperates with the sidewall of the developer housing 20 to form a supply hopper 38 with the lower portion of the electrode extending downwardly adjacent to the moving drum surface to provide a flow zone 24 therebetween through which the developer material stored in the hopper moves downwardly in contact with the drum surface. The two component developer material is stored in a reservoir 26 at the bottom of the developer housing 20. The material is elevated to a position above the hopper by means of a bucket conveyor 22 consisting of a series of parallel spaced buckets 23 secured, as by rivots, to a pair of conveyor belts 32 and the conveyor belt passed about drive pulleys 33 and 34. The conveyor is driven by motor means 36 at a speed sufficient to keep hopper 38 continuously filled with developer material when the machine is in operation.

In operation, the supply hopper 38 is of a size capable of providing the development zone 24 with a continuous downwardly moving bed of closely packed two component developer material. The term closely packed as herein used refers to a condition wherein the individual particles in the flow of material moving through the development zone are in close enough proximity to each other to inhibit the formation of powder clouds of free toner particles within the flow stream. The developer material in the moving mass, rather than being impacted against the photoreceptor surface, moves gently in contact therewith during the development process. As the developer material moves downwardly through the contact zone, toner particles are electrostatically stripped from the carrier beads and deposited in the imaged areas on the drum surface in a manner similar to that described in reference to the G shell system. The toner depleted carrier beads are then discharged from the bottom of the flow zone and returned once again to the reservoir 26.

A toner dispensing unit 35 is herein employed to accurately meter toner into the developer mix thus replenishing the toner consumed in the development process. Although any one of a number of well-known powder or granular material dispensers may be used, the toner dispenser herein employed is basically of the type disclosed by Hunt in US. Pat. No. 3,013,703. In operation, a supply of toner 37, stored in the enclosed unit, is distributed when needed into the sump or reservoir area of the developer housing by means of the metering device 29. A paddle wheel type circulating member 28 is rotatably supported in the developer housing below the dispenser and serves to redistribute the toner within the mix. The paddle wheel further provides intermixing of the two materials wherein the toner particles become triboelectrically charged and adhere to the surface of the beads in a charged state.

Although the present apparatus involves a development mechanism quite similar to that of the basic C- shell system, the flow of developer material through the active development zone is in no way dependent upon the movements of the photoreceptor surface. The geometry of the development apparatus permits a relatively high rate of developer flow to be moved through the development zone while at the same time preserving the relative gentle action of developer material against the photoreceptor surface. Because the present development mechanism is dependent primarily on electrostatic stripping rather than the formation of powder clouds and the like to accomplish development, the present apparatus is capable of developing an image upon a photoreceptor surface moving in either the uphill or the downhill direction and can be used in a relatively wide range of applications.

To further enhance the developability of the present system, backing electrode is placed at a potential to suppress background development which at the same time aids in the development of low input density images. It should be clear that the optimum operating potential is, to a large extent, dependent upon the systems parameters. It is preferred that the electrode be placed at some potential in a range between the background potential on the photoreceptor and maximum image potential on the plate. When a portion of input scene information recorded at a potential to be developed is moved through the active development zone, the higher electrostatic force field associated with the imaged areas predominates and toner material in the flow stream is readily attracted into these image areas. However, when a portion of the photoconductive surface at a background potential is moved through the development zone, the higher electrostatic force field associated with the backing electrode tends to prevent any free or loosely held toner particles from migrating into the background on non-imaged areas. Although any type bias source may be used, it preferred that the source be such as to maintain the electrode at a relatively stable DC voltage level.

A second embodiment of the present invention is illustrated in FIG. 2 in which the drum is moved in an uphill direction through the development zone. Here, two component developer material is delivered into a hopper formed by two vertically extended plates 42, 43 arranged to form a funnel capable of supporting and directing a flow of developer material into the active development zone 24. Active development zone 24 is formed, as explained above, by the cooperation of a conductive electrode 39 and rotating drum surface 11. The development zone progressively narrows from the upper introductory region (a) towards the lower exit region (b). The conductive electrode 39 is biased in a manner similar to that described above by connecting the electrode to any suitable biasing source by means of connector 31.

A wire grid 46 is also suspended in the introductory region to the development zone in the manner illustrated in FIG. 2. The grid is electrically isolated from the rest of the system and is placed at a predetermined DC. potential by means of biasing source 44 acting through electrical connector 45. The grid functions in a manner similar to an electrode and serves, in effect, to insure that the developer material passing through this region is under the influence of control field prior to contacting the drum surface. In this configuration, a

wide number of biasing arrangements can be employed. When the photoreceptor is moved in an uphill direction in relation to the developer fiow as shown in FIG. 2, the grid is placed at a potential to inhibit unwanted development in the background areas where the photoreceptor is leaving the flow zone.

Tests were conducted on the steady flow, one dimensional, system herein disclosed utilizing an arcuate shaped photoconductive surface. A backing electrode was mounted in close proximity to the moving surface and extended downwardly from approximately a horizontal position through at least 45 degrees of arc. The development zone was progressively narrowed from an entrance region spacing of approximately five thirty-seconds of an inch to an exit spacing of about three thirty-seconds of an inch. The development zone was packed with two component developer material in sufficient quantity so that the steady state one dimensional flow was maintained throughout the development cycle and the photoconductive surface moved in a downward direction while in contact with the flowing developer material. Under these conditions, development rates ranging from 4 inches per second to 60 inches per second were achieved with electrode biases ranging somewhere between volts and 350 volts, the bias polarity being similar to the image polarity on the photoconductive surface. From these tests, it was noted that developability was relatively independent of image development speeds. Commercially acceptable line-copy was made at speeds ranging from 4 inches per second to 60 inches per second. Furthermore, this system provided good low input density development while, at the same time, holding background development to a relatively low level.

While this invention has been described as referenced to the structure disclosed herein it is not confined to the details as set forth and this application is intended to cover such modifications or changes as may come within the purposes or scope of the following claims.

What is claimed is:

1. Apparatus for developing a latent electrostatic image upon an image retaining element including an extended member mounted adjacent the image retaining element, said extended member being arranged to provide a substantially enclosed, progressively more restrictive flow zone therebetween,

means to induce a flow of closely packed electroscopic developer material in said flow zone such that the flow moves from the less restrictive end thereof towards the more restrictive end in contact with the image retaining element, and

means to move the image retaining element in contact with the closely packed developer material wherein the images thereon are developed.

2. The apparatus according to claim I further including biasing means operatively connected to said extended member to place said extended member at a potential to enhance the developability of the image on said image retaining element.

3. The apparatus according to claim 2 wherein said extended member is biased to a polarity similar to that of the image retained on the image retaining element and to a potential having a magnitude greater than the potential in the non-image areas of said image retaining element but less than the potential in the image areas of said image retaining element.

4. The apparatus according to claim 3 in which said image retaining element is moved in the direction of developer flow.

5. The apparatus according to claim 3 in which said image retaining element is moved in a direction opposite to that of the developer flow.

6. Apparatus for developing a latent electrostatic image supported upon the surface of an image retaining element including a continuous electrode member mounted adjacent to the surface of the image retaining element, said member being arranged to provide a substantially enclosed, progressively more restrictive flow zone therebetween,

means to induce a flow of closely packed electroscopic developer material in said flow zone such that the flow moves from the less restrictive end thereof towards the more restrictive end means to move the image retaining element in contact with the closely packed developer material wherein the images thereon are developed, and

means to bias said continuous member to a potential to enhance development in said flow zone.

7. The apparatus of claim 6 wherein the image retaining element is moved in opposition to the flow of developer material.

8. The apparatus of claim 7 further including a biased grid positioned in the less restrictive end of said flow zone to control development in the region at which developer material first comes in contact with said moving image retaining element.

9. The apparatus of claim 8 wherein said control grid is placed at a potential to inhibit background development in the region where the image retaining element leaves the How zone.

10. The apparatus of claim 6 wherein said image retaining element is moved in the direction of developer flow.

ll 1. The apparatus of claim 6 further including means to replenish the developer material after leaving said flow zone, and

means to deliver said replenished developer material to the start of said flow zone.

12. The apparatus of claim 10 wherein said electrode member is biased to a polarity similar to the polarity of the images retained on said image retaining element and a potential between the image and background potential on said element.

13. The apparatus of claim 12 wherein said image retaining element is moved at a rate in excess of the flow rate of the developer material. 

2. The apparatus according to claim 1 further including biasing means operatively connected to said extended member to place said extended member at a potential to enhance the developability of the image on said image retaining element.
 3. The apparatus according to claim 2 wherein said extended member is biased to a polarity similar to that of the image retained on the image retaining element and to a potential having a magnitude greater than the potential in the non-image areas of said image retaining element but less than the potential in the image areas of said image retaining element.
 4. The apparatus according to claim 3 in which said image retaining element is moved in the direction of developer flow.
 5. The apparatus according to claim 3 in which said image retaining element is moved in a direction opposite to that of the developer flow.
 6. Apparatus for developing a latent electrostatic image supported upon the surface of an image retaining element including a continuous electrode member mounted adjacent to the surface of the image retaining element, said member being arranged to provide a substantially enclosed, progressively more restrictive flow zone therebetween, means to induce a flow of closely packed electroscopic developer material in said flow zone such that the flow moves from the less restrictive end thereof towards the more restrictive end means to move the image retaining element in contact with the closely packed developer material wherein the images thereon are developed, and means to bias said continuous member to a potential to enhance development in said flow zone.
 7. The apparatus of claim 6 wherein the image retaining element is moved in opposition to the flow of developer material.
 8. The apparatus of claim 7 further including a biased grid positioned in the less restrictive end of said flow zone to control development in the region at which developer material first comes in contact with said moving image retaining element.
 9. The apparatus of claim 8 wherein said control grid is placed at a potential to inhibit background development in the region where the image retaining element leaves the flow zone.
 10. The apparatus of claim 6 wherein said image retaining element is moved in the direction of developer flow.
 11. The apparatus of claim 6 further including means to replenish the developer material after leaving said flow zone, and means to deliver said replenished developer material to the start of said flow zone.
 12. The apparatus of claim 10 wherein said electrode member is biased to a polarity similar to the polarity of the images retained on said image retaining element and a potential between the image and background potential on said element.
 13. The apparatus of claim 12 wherein said image retaining element is moved at a rate in excess of the flow rate of the developer material. 